This invention relates to a process for the removal of H.sub.2 S from molten sulfur. More specifically this invention relates to the removal of residual H.sub.2 S from molten sulfur by the addition to the molten sulfur of one or more compounds described hereinafter which promote the removal of H.sub.2 S and hasten the decomposition of hydrogen polysulfides to H.sub.2 S. Hastening this decomposition permits convenient disposal of the released H.sub.2 S before the sulfur is shipped by the manufacturer.
During the manufacture of elemental sulfur, particularly by the reaction of: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+H.sub.2 O (1)
small quantities of H.sub.2 S and hydrogen polysulfides, typically 150-300 parts per million by weight (wppm), may remain in the molten sulfur. During storage the hydrogen polysulfides decompose to hydrogen sulfide, which is slowly released by the sulfur. This slow release of hydrogen sulfide is undesirable for several reasons. Since the threshold odor limit of H.sub.2 S in air is relatively low, approximately 0.1 parts per million by volume (vppm), this slow release may create nuisance odors. Moreover, H.sub.2 S is highly toxic, flammable and may form explosive mixtures with air. Accordingly, the presence of H.sub.2 S in molten sulfur is being limited in many locations by increasing stringent regulations.
Several methods have been used to decrease the residual hydrogen sulfide and hydrogen polysulfides concentration in sulfur. One method has been merely to allow a "weathering off" process to take place before the sulfur is transported. But, this is a relatively slow process that requires large sulfur storage capacity.
A method for increasing the rate of this "weathering off" process uses a circulatory spraying system. However, this requires the purchase and use of additional circulatory equipment and may not reduce the residual hydrogen sulfide and hydrogen polysulfide to sufficiently low levels.
Other methods have involved the addition of chemical compounds and sulfur dioxide to the molten sulfur to convert the hydrogen sulfide and hydrogen polysulfides to elemental sulfur. British Pat. No. 1,393,967 describes a method for reducing the amount of hydrogen polysulfides and hydrogen sulfide by in situ reaction to form elemental sulfur. Sulfur dioxide and an additive, which is an inorganic alkali metal compound, ammonia, an ammonium compound or a basic organic compound with at least one amino group, is injected into the liquid sulfur while the sulfur is flowing through a conduit. Table I of this patent indicates that large amounts of H.sub.2 S remain in the molten sulfur when SO.sub.2 is not added together with the additives noted. U.S. Pat. No. 3,447,903 discloses the addition of certain classes of compounds together with SO.sub.2 to molten sulfur for a variety of purposes, including the removal of small quantities of hydrogen sulfide. Among the classes of compounds disclosed are:
(A) Ammonia and its organic derivatives having a K.sub.B greater than 10.sup.-10 ;
(B) Primary, secondary and tertiary alkyl, aryl and cyclic amines having a K.sub.B greater than 10.sup.-10 ;
(C) Quaternary basic nitrogen compounds that decompose between 20.degree. and 160.degree. C. to yield ammonia or its inorganic derivative having a K.sub.B value greater than 10.sup.-10 ; and
(D) Quaternary basic nitrogen compounds that on heating to temperatures between 20.degree. and 160.degree. C., decompose to yield an organic amine having a K.sub.B value greater than 10.sup.-10. However, this process requires the further addition of sulfur dioxide to react with the H.sub.2 S in place and form sulfur rather than liberating the H.sub.2 S.
Use of SO.sub.2 to remove H.sub.2 S from molten sulfur is not desirable for several reasons. Sulfur dioxide is difficult to store, since it is gaseous at ambient conditions. Thus, means must be provided for injecting the SO.sub.2 directly into the molten sulfur and for adjusting the relative quantities of SO.sub.2 and the other additive metered into the molten sulfur. In addition, SO.sub.2 is toxic, a strong irritant to the eyes and mucous membranes and an air contaminant. Moreover, SO.sub.2 is corrosive to the carbon steel surfaces of some sulfur handling facilities. These factors may complicate or restrict its use in commercial facilities.
U.S. Pat. No. 3,278,269 describes several classes of compounds which may be added to molten sulfur to improve the friability of solidified sulfur. The compounds disclosed include the following:
A. Liquid and solid primary, secondary and tertiary alkyl, aryl, and cyclic amines having a K.sub.B value greater than 10.sup.-10 in which the amino nitrogen is attached to a primary, secondary or aromatic hydrocarbon;
B. Quaternary nitrogen compounds that decompose between 20.degree.-160.degree. C. to yield ammonia; and
C. Quaternary nitrogen compounds that on heating to temperatures between 20.degree. and 160.degree. C. decompose to yield an organic amine having a K.sub.B value greater than 10.sup.-10 in which the carbon atom directly attached to the amino nitrogen is primary, secondary or aromatic.
U.S. Pat. No. 3,364,655 discloses that H.sub.2 S can be removed from liquid sulfur by atomizing the sulfur, and that this process can be further improved by the addition of ammonia.
In Z. Anal. Chem. 166 (1959) pages 274-283 Schmidt and Talsky describe a method to convert hydrogen polysulfides to hydrogen sulfide by the addition to the sulfur of sulfites or cyanides.
It is desirable to provide a process in which the residual hydrogen sulfide and hydrogen polysulfides are reduced to relatively low levels without the use of excessively large, or corrosion resistant storage facilities.
It is also desirable to provide a process in which the hydrogen sulfide and hydrogen polysulfides can be reduced to relatively low levels in the molten sulfur by the addition of degassing agents that do not require complex addition or metering facilities.
It is also advantageous to provide a process in which the hydrogen sulfide and hydrogen polysulfides are reduced to lower levels by the addition of chemical compounds that do not significantly increase the ash content of the product sulfur and may, in certain instances, reduce the product sulfur ash content.